At present, the standards of third generation (3G) mobile communication systems mainly include the Wideband Code Division Multiple Access (WCDMA) protocol proposed by Europe and the Code Division Multiple Access 2000 (CDMA 2000) protocol proposed by North America. Both of these protocols define and describe in detail radio transmission technology, channel structure and network signaling, etc., used in the systems Among these, the WCDMA provides in down link synchronization channel (SCH), dedicated physical channel (DPCH), common control physical channel (CCPCH), dedicated shared channel (DSCH) and acquisition indicator channel (AICH), etc. as down link physical channel. It also applies dedicated pilot technology, i.e., it applies pilot symbol and data information time multiplexing mode for DPCH, CCPCH and DSCH where pilot is needed.
Under this mode, a mobile station uses pilot symbols to estimate the channel, and the estimation result is used to demodulate the subsequent data. When a mobile station moves quickly, the channel parameters change rapidly and can cause the result of demodulation to be inaccurate. Very complicated estimation methods can be required in order to provide a better estimation result, and such methods are very difficult to implement by hardware.
In addition, as there is no independent pilot channel, a mobile station can only do physical layer measurements for cell selection/reselection, handover and power control etc. on CCPCH. This is sometimes very inconvenient. For example, as CCPCH also carries other information of a base station, pilot symbols are not transferred continuously. When it is necessary to accumulate pilot symbol energy, accumulation control becomes more complicated, and suppressing the inference of noise becomes more time consuming.
Other systems use time division mode of pilot and data so that data included in each channel is decreased and the data rate is lowered accordingly.
In the CDMA 2000 system, all of the base stations are synchronized. In down link, a common continuous pilot channel, i.e. Forward Pilot Channel (F-PICH), is provided. The F-PICH is shared by all traffic channels. It is transmitted in the cell continuously and used for channel estimation, multiple path detection and cell acquisition and handover. All data transmitted on F-PICH are “1” and modulated on a Walsh code, and different base stations have different phases of Walsh code. With the Walsh code, a mobile station achieves initial synchronization with the “most powerful” base station and at the same time, other channels do not transmit the pilot signal anymore.